1. Field of the Invention
This invention relates to a process for preparing oligomer of at least one polyunsaturated aliphatic C.sub.12-22 monocarboxylic acid ester in the presence of boron trifluoride resulting in a high yield of predominantly trimeric product with excellent recovery of the boron trifluoride.
2. Description of the Related Art
The oligomerization of esters of unsaturated monocarboxylic acids in the presence of boron trifluoride (BF.sub.3) to produce mixtures of dibasic acids, also known as dimer acids, and polybasic acids, also known as trimer or higher acids, is well known. For instance, Croston et al. "Polymerization of Drying Oils. VI. Catalytic Polymerization of Fatty Acids and Esters with Boron Trifluoride and Hydrogen Fluoride", Journal of the American Oil Chemists' Society, 331-333 (Aug., 1952) describes the polymerization of soybean fatty acids as well as their methyl esters in the presence of boron trifluoride. Typically, use of 2% BF.sub.3 as a catalyst at 150.degree.-200C. resulted in the polymerization of 50-60% of the methyl esters within one hour. BF.sub.3 catalysis resulted in a low ratio of dimers to higher polymers, although the product distribution was not well-characterized.
Ghodssi et al. "Cationic Polymerization of Oleic Acid and its Derivatives. Study of Dimers", Bulletin de la Societe Chimique de France No. 4: 1461-1466 (1970) describes the synthesis of methyl cleate dimers, including higher polymeric by-products, by cationic polymerization of methyl oleate by bubbling BF.sub.3 through the monomer at about 20.degree.-30C. Molecular distillation gave two fractions which were characterized as the monomer (53.6 w/o) and dimers (18 w/o), as well as a residue (23.2 w/o) which was characterized as oligomers with a degree of oligomerization greater than 2. Here and throughout this application measurements given in percent mean weight percent (w/o) unless otherwise noted.
Dimer and trimer acids produced by oligomerization of unsaturated fatty acids (e.g., tall oil) have a variety of commerical and industrial uses. Dimer acids are used in solid and liquid polyamide resins, urethane resins, corrosion inhibitors, maintenance paints, varnishes, adhesives, soaps, polymer modifiers, oil additives, and lubricants. Trimer acids or their amine derivatives are particularly useful as a corrosion inhibitor/rust preventative in drilling mud formulations for the oil drilling industry, as a flexibilizing curing agent for epoxy resin coatings, and in soap-based lubricating greases. Most dimer and trimer acids sold commercially are actually mixtures of the two in which the named acid predominates. However, quite pure dimer and trimer acids are separable by molecular distillation and are available commercially. In fact, pure trimer acids are presently commercially obtained as by-products of the purification of dimer acids by molecular distillation. Unfortunately, the often limited demand for molecularly distilled dimer acids results in a limited production of the trimer acid by-product which is insufficient to satisfy the demand for it. Thus, in the past, a number of attempts have been made to produce trimer acid, instead of dimer acid, as the major reaction product. These prior art oligomerization reactions have involved a variety of catalysts, including modified montmorillonite clays, zeolites, peroxides or hydroperoxides (for free radical-initiated polymerization reactions), and strong acids such as p-toluenesulfonic acid. However, these catalysts produced insufficient yields of trimer acids and/or undesirable by-products.
Oligomerization of linoleic acid with 3-5% BF.sub.3 is known to give a trimer acid product having a low saponification number (about 180) and acid value (about 160) resulting from formation of interesters, which are by-products resulting from the unwanted reaction between the carboxyl function and the alkenic function of the starting material. Further reaction over montmorillite clay to reduce the interester results in a trimer acid product of poor quality.
The use of boron trifluoride, a Lewis acid, as a Friedel-Crafts-type catalyst is well known. For instance, U.S. Pat. No. 3,000,964, issued to J. G. Milligan, describes a process using BF3 catalysis in the alkylation of phenols. U.S. Pat. No. 3,929,749, issued to T. A. Cooper and A. L. Logothetis and references cited therein, discusses the use of BF3 catalysis in the production of alternating copolymers of ethylene and alkyl acrylates. In the cationic polymerization of unsaturated carboxylic acids and esters to yield polybasic acid products, mentioned hereinabove, BF.sub.3 is known to be advantageous because it catalyzes the reaction at low temperatures, thereby minimizing the degradative by-products which occur when catalysts requiring higher temperatures are utilized. In order to achieve desirable reaction rates, however, these syntheses require substantial, and often greater than stoichiometric, amounts of BF.sub.3 due to initial complexation of the catalyst with starting material. Because BF.sub.3 is relatively expensive, such processes are not economically practicable without the recovery and recycling of the BF.sub.3.
BF.sub.3 may be removed from the reaction mixture in several ways. Some techniques are degradative and therefore not of interest. For instance, steam stripping of an alkyl acrylate copolymer solution containing BF.sub.3 complexed to the alkyl acrylate moiety allows isolation of uncomplexed copolymer but the resulting hydrolysis of the BF.sub.3 precludes its recovery and recycling. However, a number of removal techniques are known which do not degrade the BF.sub.3.
Pyrolysis removes BF3 by heating the reaction mixture to elevated temperatures and holding while the gaseous catalyst is evolved. However, such heating can cause undesirable degradation of the reaction product.
Several solvent stripping techniques for BF3 removal are also known. U.S. Pat. No. 3,929,749 describes a process wherein superheated solvent is passed into a pressurized heated reactor containing a BF.sub.3 -complexed ethylene-alkyl acrylate copolymer solution and BF.sub.3 -complexed alkyl acrylate. Using sufficient pressure to keep the solvent from volatilizing, the solution is heated sufficiently to dissociate the complexes and to liberate the BF.sub.3. The whole mixture is then passed into a flash chamber, held at atmospheric or subatmospheric pressure, causing the BF.sub.3 to flash off along with the solvent. In U.S. Pat. No. 3,000,984, process for removing BF.sub.3 from a phenol alkylation mixture is described. An inert paraffinic or aromatic hydrocarbon entrainer in which BF.sub.3 is sparingly soluble, having a boiling point within the range of 30.degree.-200C. at atmospheric pressure, is added to the reaction mixture. In general, from 30-200 weight percent (w/o) entrainer (relative to reaction mixture weight) is required. The mixture is heated until the entrainer boils, the entrainer is condensed and reused, while the gaseous BF3 is removed and recycled by complexation with phenol starting material.
Similarly, U.S. Pat. No. 4,017,548, issued to D. G. Petrille describes another process for the recovery of BF.sub.3 from a phenol alkylation mixture by solvent stripping. An inert alkane hydrocarbon with a boiling point within the range of 80.degree.-125C. is continuously dissolved in the reaction mixture in an amount of 6-25 w/o, relative to the reaction mixture. The resulting mixture is continuously heated in incremental portions to 100.degree.-130C., which causes the BF.sub.3 phenolate complex to dissociate and the alkane to vaporize. The alkane vapors strip the BF.sub.3 out of the reaction mixture, thereby liberating gaseous BF.sub.3 which is recycled for use by complexation with the phenolic starting material. While effective in removing BF.sub.3, such solvent stripping techniques have the disadvantage of requiring relatively large quantities of solvent, thereby increasing solvent and processing costs.